4-Methyl-2-oxetanone, also called .beta.-butyrolactone or .beta.-methyl-.beta.-propiolactone, has been prepared by reaction between ketene and acetaldehyde as disclosed in JP-B-47-25065 (the term "JP-B" as used herein means an "examined published Japanese patent application") or hydrogenation of 4-methylene-2-oxetanone, also called diketene, in the presence of palladium black as disclosed in U.S. Pat. No. 2,763,644.
While 4-methyl-2-oxetanone has been used, for example, as a starting material of polymers (see Yuya Yamashita, et al., KOGYO KAGAKU ZASSHI, Vol. 66, No. 1, pp. 110-115 (1963)), attention is now given to the usefulness of its optically active form as reported by N. Tanahashi, et al., Macromolecules, Vol. 24, pp. 5732-5733 (1991).
The above-mentioned conventional processes only produce a racemic modification of 4-methyl-2-oxetanone. Processes for producing optically active 4-methyl-2-oxetanone which have been proposed to date include:
1) a process comprising optically resolving 3-bromobutyric acid, which is obtained by addition of hydrobromic acid to crotonic acid, by using optically active naphthylethylamine followed by cyclization (see J. Reid Shelton, et al., Polymer Letters, Vol. 9, pp. 173-178 (1971) and T. Sato, et al., Tetrahedron Lett., Vol. 21, pp. 3377-3380 (1980)), PA1 2) a process comprising reacting optically active 3-hydroxybutyric acid with triethylorthoacetic acid to obtain optically active 2-ethoxy-2,6-dimethyl-1,3-dioxan-4-one, which is then thermally decomposed (see A. Griesbeck, et al., Helv. Chim. Acta, Vol. 70, pp. 1320-1325 (1987) and R. Breitschuh, et al., Chimia, Vol. 44, pp. 216-218 (1990)), and PA1 3) a process comprising reacting an optically active 3-hydroxybutyric ester with methanesulfonyl chloride to mesylate the hydroxyl group, hydrolyzing the resulting ester, followed by condensation and cyclization using sodium hydrogencarbonate (see Y. Zhang, et al., Macromolecules, Vol. 23, pp. 3206-3212 (1990)).
These known processes for preparing optically active 4-methyl-2-oxetanone have their several disadvantages as follows.
Process (1) not only requires a special optically active amine as a resolving agent in an equimolar amount with the starting compound but by-produces an unnecessary enantiomer in an equimolar amount with the purposed isomer. Therefore, this process involves much waste and is uneconomical.
Processes (2) and (3) encounter difficulty in synthesizing the starting compound, i.e., optically active 3-hydroxybutyric acid or an ester thereof. That is, the synthesis of these compounds involves many steps and complicated operation, such as thermal decomposition of an optically active poly-3-hydroxybutyric ester produced by a microorganism, or subjecting racemic 4-methyl-2-oxetanone to alcoholysis to once obtain an acetoacetic ester, which is then asymmetrically reduced.
Hence, it has been keenly demanded to develop a process for preparing optically active 4-methyl-2-oxetanone which is easy to carry out and economically advantageous.